The invention relates to an electron gun of a cathode ray tube. More particularly, this invention relates to a main lens of the electron gun for reducing flare, coma and improving the spot shapes of electron beams emitted therefrom.
The main lens structure of a color cathode ray tube (CRT) electron gun generally includes a pair of grid structures for focusing emitted electron beams along desired axes. Generally, the first grid structure has a first electrical potential or xe2x80x9cfocus voltage,xe2x80x9d the second grid structure has a second electrical potential or xe2x80x9cscreen voltagexe2x80x9d of the CRT. These grid structures each include a similar pattern of three apertures. The apertures of each grid electrode are substantially aligned and centered about a desired focal axis.
Each aperture of a grid is provided for receiving a corresponding electron beam (red, green, blue) from the electron gun. The grid electrodes are often recessed into a larger common-lens structure, which, while reducing main-lens aberrations, also introduce differences between the focus characteristics of the center (centermost electron beam) and outer guns (outermost electron beam).
Aberrations are key determining factors in assessing electron gun spot performance (i.e, shape of the electron beam point contacting the display surface). In particular, aberrations can lead to spot flare, especially in the horizontal direction where beam sizes are usually the largest. In the outer-gun spots aberrations lead to unbalanced flare, which leaves the spot with a triangular shape.
Known aperture shapes employed by grid electrodes are circular and elliptical. The outer apertures frequently consist of pairs of semi-ellipses, with common heights but differing half-widths. Diamond shaped apertures are also known. Aperture shapes, whether elliptical or diamond shaped are designed with horizontal and vertical dimensions in mind. Since these dimensions are primarily used to set horizontal and vertical focus voltages, known shapes provide little if any control over how the aberration content of the lens varies as a function of angle around the aperture center.
Accordingly, there is a need for a new aperture shape for reducing focal aberrations.
The present invention comprises a main lens of an electron gun of a cathode ray tube. The main lens receives a plurality of substantially parallel electron beams emitted by a beam-forming region. The lens focuses each electron beam along a respective one of a plurality of focal axes incident to a display surface. A first grid electrode is positioned substantially orthogonally with respect to the plurality of electron beams. The grid electrode includes a plurality of apertures, each aperture focuses a respective one of the plurality of electron beams. Each aperture is centered about a respective one of said focal axes and has a shape expressed by the equation (1):                                                                         (                                  x                  a                                )                                            2                n                                      +                                          (                                  y                  b                                )                                            2                n                                              =          1                ⁢                  
                ⁢        or        ⁢                  "IndentingNewLine"                ⁢                              x            =                          a              ⁢                              xe2x80x83                            ⁢                              cos                n                            ⁢                              xe2x80x83                            ⁢              θ                                ;                      xe2x80x83                    ⁢                      y            =                          b              ⁢                              xe2x80x83                            ⁢                              sin                n                            ⁢                              xe2x80x83                            ⁢              θ                                                          (        1        )            
where the origin (x=0, y=0) is at the center of the aperture, the x-axis is a line from the center of the aperture to the center of an adjacent aperture and the y-axis is perpendicular to the x-axis. The terms a and b define the horizontal and vertical axis half-widths (distance from the origin to the edge of the aperture), xcex8 is the angle formed with respect to the x axis by a line drawn from the origin to a point on the aperture and ranges from 0xc2x0 to 360xc2x0. The exponent n determines the deviance from ellipticity, where 1 less than n less than 2. The shape of the plurality of apertures diminishes focal aberrations of the lens.
In another aspect of the present invention, a main lens of an electron gun of a cathode ray tube is provided wherein differential values of n are used for the two halves of the outer apertures and those halves of n may, in turn, differ from that of the center aperture so that the shape of each aperture reduces main lens focal aberrations.